CONIFERALES (PINACEAE) 



275 



and the pollen, caught in this pollination drop, is conveyed to the 

 nucellus (fig. 315). The pollen tube begins to grow into the nucellus 

 as soon as the spore is deposited, and continues to develop until it is 

 checked by cold weather. The next spring the tube begins to renew 

 its penetration of the nucellus during April, about a year after the 

 pollen mother cell entered upon the reduction divisions, the large 

 tube nucleus enters the tube, and at the same time the generative 

 cell divides into the stalk cell (toward the vegetative cells) and the 

 body cell (toward the free cavity of the spore) (fig. 313). The 

 stalk cell is sterile (persistently 

 so among gymnosperms), and 

 may represent the stalk cell of 

 an antheridium, while the body 

 cell may be the true primary 

 spermatogenous cell. The 

 pollen tube branches as it 

 traverses the nucellus, not so 

 extensively as do the tubes of 

 cycads and of Ginkgo, but 

 sufficiently to indicate its primi- 

 tive haustorial function. After 

 its second start, the pollen 

 tube consumes about two months in traversing the nucellus, reaching 

 the archegonium about the first of July. 



The body cell rounds off, and becoming freed from the stalk cell 

 passes into the tube. The separation of the body cell from the stalk 

 cell frees the nucleus of the latter, and it also passes into the tube, 

 where the nucleus of the body cell divides, forming two nuclei which 

 do not become separated by a wall. The body cell, with its two 

 nuclei, then passes down the tube, and when it has reached the tip, 

 the four nuclei may be found grouped together, usually with the 

 stalk and tube nuclei in advance. At this stage the contour of the 

 body cell may be quite distinct (fig. 316), or rather indefinite (fig. 317), 

 or its cytoplasm may have become so mingled with that of the pollen 

 tube that the nuclei lie in a common mass of cytoplasm (fig. 318). 



The division of the body cell occurs just before fertilization, about 

 two months after the division of the generative cell into stalk and body 



Fig. 315. — Cephalotaxus Fortunei: pol- 

 lination drops at the tips of the numerous 

 ovules; X3.33. — ^After TisON (171). 



